Submarine optical communication cables are optical cables laid on the sea bed connecting land-based stations. The optical communication cables carry signals across the ocean allowing for cable communication over the sea between different continents. Each transoceanic submarine optical cable includes multiple (up to eight) pairs of fibers; each pair has one fiber in each direction. The submarine optical communication cable is divided into multiple sections, each section connected to the other section by a submarine repeater. As the signals are carried across the ocean, the signals may lose some of their power. Each submarine repeater comprises multiple Erbium Doped Fiber Amplifiers (EDFA), one amplifier for the signal in each direction of each fiber. Each EDFA has a gain sufficient to compensate for the loss experienced by the signal during its propagation in the previous section of fiber. A typical length of cables (i.e., fiber) section between repeaters is about 60 km. Atypical Trans-Pacific cable with a length of 10,000 km has about 150-180 repeaters. Therefore, the repeaters ensure that the land-based station receiving the signal, from another land-based station, understands the signal. In other words, the repeaters increase the power of the signal to compensate for any loss during the signal propagation. The increased power translated to higher capacity because the clearer and more precise the optical signal is, the more complex signals may be sent from one land-based station to another.
Referring to FIG. 1, an optical communication system 10 includes first and second trunk terminals 110, 120 (also referred to as stations) coupled to a communication trunk 102. The communication trunk may include one or more repeaters 150. The repeaters 150 are powered by a constant current, typically 1 Ampere by power feeding equipment, e.g., power source 112 located by the shore. Due to nonzero electrical resistivity of copper, even with large copper area conductor having a resistance as low as 1 Ohm/kilometer, power feeding voltage drops by 60 Volt at each section of cable, so that about half of power feeding voltage is lost due to heat dissipation in copper for Trans-Pacific cable. In some examples, each power source 112 can provide power feed voltage of up to 15 kilovolts. Further increase of power feeding voltage to higher that 15 kV may result in a fault during cable operation, which has a lifetime of 25 years. The power feeding equipment 112 powers the repeaters 150 by a power cable 113, such as a copper cable. With half of power feeding voltage lost due to heat dissipation in copper cable, and due to large number of repeaters, 150-180, voltage drop at each repeater is limited to below 50 Volt. Typical output power launched into a submarine fiber is 17 dBm (50 mW) for each direction. Assuming highly efficient conversion from electrical into optical of 30% in diode laser and 10% efficiency of EDFA pumped by diode laser, each EDFA requires about 2 Volts power drop at constant current of 1 A. Thus 50V power feed voltage per repeater limits the number of EDFAs in the repeater to 20-25, i.e., supporting compensation of loss for not more than 10-12 fiber pairs. Most of subsea cables today have six fiber pairs. Thus, the power feeding of repeaters from the shores limits further growth of capacity of trans-oceanic submarine cables.